What I did is I looked at the overall competitiveness of the firm, in terms of getting a job there.
I also talked to many people in the industry, as well as at hedge funds and investment banks - I asked them about which firms were most reputable in terms of order of operations, compensation, and training. I talked to traders at many of the firms to get a sense of their own personal thoughts on the culture, technology, management, and past/future success of the firms.
In addition, I've interviewed with most of the firms on that list and I ranked them based off of my own judgment to some degree.
It also depends on what you want to trade- equities, options, futures, rates, f/x, ect.
Many of those firms focus on a specific product area and a specific style (whether it be market making or discretionary trading).
When you're picking which firm to go with though - the key, in my opinion, is to pick a firm with a winning business model and winning strategies. Firms look for talented and bright candidates that they feel will be able to execute their strategies well....
so I basically tried to judge which firms have the winning-est business models and strategies.
Obviously the list is just my opinion... and any comments or thoughts would be great.